Solder

So, you’ve selected your soldering system, have your tips on hand, learned a little about what flux to use, and now you’re asking what type of solder would be best for your own project or needs.  “That’s a very good question.”  Let’s see if I can shed some light on this. 

First of all let’s go over what solder is made from and then we’ll work on what you may need. 

Solder is basically a filler material for soldering parts together.  We will only cover electronic applications as there are other types of solder alloys for use in jewelry making, brazing pipes, and many other industries outside of electronics.

Solder is available in many alloys for differing applications.  In electronics, the alloy or “Eutectic alloy”, of 63% tin and 37% lead (or 60/40) which is close or almost the same in its melting point, has always been the choice that most technicians prefer.  “I use this mix on a daily basis in my own lab.”

This mix of tin and lead has its advantages.  When the solder is heated to its melting point the solder will flow smoothly at its lowest temperature.  This prevents the solder from going through what is known as the “plastic phase”.  Having the lowest possible melting point prevents heat stress to electronic components.  With little to no heat stress this prevents possible cracking of the electronic components. With no “plastic phase” present this allows for quicker wetting as the parts heat up and faster setup times as the solder cools.

Solder in roll form

Any “non-eutectic” formulations of solder, like lead-free solder, can result in unreliable joints if the parts don’t remain still during the soldering process. 

Some common formulations for solder that I use and have used in my own lab are shown below.

63/37 – This melts at around at 183  Degrees Celcius or 361 Degrees Fahrenheit  (eutectic: the only mixture that melts at a point, instead of over a range)

60/40 – This melts around 183-190 Degrees Celcius or 361-374 Fahrenheit 

50/50 – This melts around 183-215 Degrees Celcius or 361-419 Degrees Fahrenheit

So, as you can see the different mixes of solder generally melt within similar temperature ranges.  There are not a lot of differences in the formulations.

Some solders don’t contain lead at all.  These “lead-free” solders are more widely used in countries that adhere to lead-free restrictions for electronics or rather (RoHS) or “Restriction of Hazardous Substance Directive.”  This directive mostly covers European Countries and is not widely used in the United States or elsewhere. 

These lead free solder formulations were introduced in an attempt to lower the use of lead in the electronics industry.  It was thought that by using lead free solder the amount of lead that found its way into landfills, toys, and groundwater from discarded electronic devices would or could be reduced.  Unfortunately, for the electronics industry, the use of lead-free solder alloys has made it difficult to produce quality electronic devices.  Lead free solder does not contain any eutectic formulations.  This type of solder melts at around 250 Degrees Celsius or 482 Degrees Fahrenheit.  This high temperature needed to melt the solder makes it very difficult to create a reliable joint.

Some other common solders include low temperature formulations.  Some of these alloys contain “Bismuth”.  

 One type I use to help remove stubborn circuit board components is called “Chip Quik”.  This solder alloy melts at a very low temperature and is available in leaded and lead free formulations.   

Here is a quick video on how Chip Quik works

Some solders melt at a much higher temperature.  One such solder is “silver solder”.  This alloy is used where high strength joints are desired where common solders will just not work.  Silver solder is also used to solder assemblies that you don’t want to become unsoldered during rework of surrounding components.  There are also specialty solder formulations for soldering aluminum components. 

Solder comes in many forms as well.  The most common method of using solder is on rolls.  However, there are also bars and paste solder.  I purchase bar solder for my lab solder pots.  This is an actual heating pot made specifically for melting solder.  I use one when I have many wire ends to tin or leads that need tinned on a large scale.  You may want one for your own use if you solder and need to frequently tin wire ends or leads.

Solder sizes are as varied as there are types, from the very small of just .010” to the larger .125” for common rolls.  You can purchase bars as well in leaded and lead-free formulations. 

Solder wire also comes in some of these common formulations:

(10/88/2), (60/40), (62/36/02), (62/36/2), (63/37), (Classic Tin/Lead), (Sn60),( Sn62), and (Sn63/Pb37)

For a complete breakdown of each designation, refer to the manufactures specifications.   Solder can also come with no flux core or be manufactured containing no-clean,  rosin, mildly activated rosin, water soluble, Glow core no-clean, or activated Rosin.  

 Refer to my article on flux for more information on fluxes and how they work.

The last type of solder I want to cover is “solder paste”.  Solder paste is a mixture of solder powder and flux.  The flux acts as a tacky, viscous binding agent to help hold the solder in place as it is melted.  The wetting action of the flux as it’s heated helps to flow the solder.  This type of solder is used mostly for BGA and LBGA solder ball applications.  To put in laymen’s terms, it’s used for the types of chips you would find on a computer circuit board that has no physical leads showing.  The solder pads are hidden underneath the component. Usually manufacturers utilize stencils when using solder paste.  However, you can use solder paste on your own project if you follow the example shown below.

Simple video showing how to use solder paste

I personally do not like using solder paste.  The small solder balls mixed in the flux have a tendency to roam if not heated properly.  You’re more susceptible to solder bridges and getting solder under the leads.  So, I say stick to standard tin/lead solder for your home projects. 

Selecting the right size will be up to you.  I generally use a solder gage that is small enough to fit the size of the wire or component pad.  Anything larger and you risk using too much.  This usually results in a large glob of solder on the worked item.  I use .010 for soldering extremely small components such as a 0402 or 0603 SMD (surface mount device).   .015 works well for 0805 to 1206 SMD components.  For tinning wire I either use the solder pot for multitudes, or start at .025 and work my way up from there depending on the wire size. 

Whatever you use will ultimately be up to what you decide works best for your own project.  What I have presented to you in this article should help lay a good foundation for your own soldering selection and experience.